Fix for coding-system completion (bug#23670)

* lisp/international/mule.el (read-buffer-file-coding-system):
Ensure that completion-pcm--delim-wild-regex is enclosed in parens,
so that completion-pcm--pattern->regex can append "*?".

lisp/international/mule.el

@@ -1311,8 +1311,8 @@ Internal use only.")
                                     preferred))))))
          (completion-ignore-case t)
          (completion-pcm--delim-wild-regex ; Let "u8" complete to "utf-8".
-          (concat completion-pcm--delim-wild-regex
-                  "\\|\\([[:alpha:]]\\)[[:digit:]]"))
+          (concat "\\(?:" completion-pcm--delim-wild-regex
+                  "\\|\\([[:alpha:]]\\)[[:digit:]]\\)"))
          (cs (completing-read
               (format "Coding system for saving file (default %s): " default)
               combined-table